Two-step method for middle distillate hydrotreatment comprising two hydrogen recycling loops

ABSTRACT

A process for hydrotreating a hydrocarbon feed comprises at least two steps with intermediate stripping of the effluent from the first step with pressurized hydrogen substantially free of impurities which are undesirable for the catalyst for the second step, in particular to eliminate part of the H 2 S formed, each step being carried out with a hydrogen recycle loop that is exclusive to that step.

The present invention relates to the hydrotreatment of hydrocarbonfractions, for example gasoline or middle distillates, to producehydrocarbon fractions with a low sulphur content, low nitrogen contentand low aromatics content, particularly for use in the field of internalcombustion engine fuels. Such hydrocarbon fractions include jet fuel,diesel fuel, kerosene and gas oils. The invention can process middledistillate cuts with a very low cetane index, for example cutscomprising a high proportion of light cycle oil, LCO, to produce a fuelwith a high cetane index, which is desulphurized and partiallydearomatized.

Currently, middle distillate type cuts from straight run distillation ofa crude oil or from a catalytic cracking process still contain nonnegligible quantities of aromatic compounds, nitrogen-containingcompounds and sulphur-containing compounds. Current legislation in manyindustrialized states requires that fuel for use in diesel engines mustcontain less than about 500 parts per million (ppm) of sulphur, and thatspecification must soon be reduced to 50 ppm; very probably it will inthe medium term be further reduced to 10 ppm or even less. Currently,the aromatics content of a diesel fuel is not regulated, but in somecases the amount of aromatics in base cuts must be limited in order tosatisfy cetane index specifications.

Thus, changes in specifications render necessary the development of areliable, efficient process for producing, from conventional straightrun middle distillates or from catalytic cracking (LCO cuts) or fromanother conversion process (cokefaction, visbreaking, residuehydroconversion, etc), a product with improved characteristics both asregards the cetane index and as regards the aromatics or nitrogencontent, and in particular the sulphur content.

The present invention concerns a high performance hydrotreatment processthat can in particular be used to treat difficult feeds of mediocrequality to produce high quality fuels. It comprises at least tworeaction steps with intermediate stripping of the effluent from thefirst step with pressurized hydrogen substantially free of impuritieswhich are undesirable for the catalyst for the second step, inparticular to eliminate part of the H₂S formed, each step being carriedout with a hydrogen recycle loop that is exclusive to that step.

That process means that the operational pressures for the two steps canbe selected independently to have a very low pollutant level in thesecond step, in particular of H₂S and water, and to be able to use acatalyst based on a noble metal or comprising a noble metal in thesecond step under the best service conditions for that catalyst, allwith high energy efficiency.

The present invention also concerns a substantially desulphurizedhydrocarbon fraction that is possibly partially dearomatized obtainedusing the process of the invention, and any fuel containing saidfraction.

DEFINITIONS AND CONVENTIONS USED IN THE INVENTION

The present description of the invention will use the followingnotations, definitions and conventions:

-   -   ppm for parts per million, expressed by weight;    -   conventionally, the pressure of a reaction step is the pressure        at the outlet from the reactor (or last reactor) of that step.        Conventionally, the pressure of a hydrogen-rich recycle loop is        the pressure at the intake of the recycle compressor;    -   the term “purity” as applied to a hydrogen-containing gas and        designated “Pur” means a molar percentage of molecular hydrogen:        as an example, a purity Pur1 of 85 for a gas means that the gas        contains 85 mole % of hydrogen (molecular). Conventionally, a        “hydrogen-rich gas” or “hydrogen” is a gas with a molecular        hydrogen purity of more than about 50. Conventionally, the        purity of a hydrogen-rich recycle gas (or purity of the gas in        that loop) is the purity of the gas at the intake of the recycle        compressor for that loop;    -   the term “recycle loop” or “hydrogen recycle loop” is applied to        recycling a hydrogen-rich gas to a reactor after downstream        gas/liquid separation and compression of the gas (or a portion        of the gas) to allow recycling. By extension, a “recycle loop”        also comprises the lines and equipment traversed by the recycle        gas, possibly mixed with a liquid phase (such as the feed). In        particular, a recycle loop comprises at least one recycle        compressor, a hydrotreatment reactor, a gas/liquid separation        drum downstream of the reactor and/or optionally, the portion of        the pressurized hydrogen stripping column located above the        supply, if that column is located on the recycle gas circuit:        thus, a recycle loop comprises the lines and equipment located        on the path of the recycle gas; it can thus also comprise        branches and bypasses: as an example, a portion of the        compressed gas can be removed from the stream of recycle gas        upstream of the reactor and supply the reactor at an        intermediate portion for use as a quench gas or as the stripping        gas. The term “recycle loop” is thus used to qualify a closed        circuit that can comprise, downstream of the recycle compressor,        branches and parallel portions of the circuit, following the gas        path, said portions generally joining together upstream of the        recycle compressor or compressors for that loop. In contrast,        open circuits are excluded from the term “recycle loop”, such as        circuits without recycling, or circuits for evacuating gas, for        example to other facilities, for example circuits for evacuating        gas taken from a recycle loop such as an excess gas or purge        gas;    -   the feed for the process of the invention designates a liquid        stream of hydrocarbons supplying a hydrotreatment zone and also        a liquid stream recovered downstream of said hydrotreatment zone        which can, for example, be termed a partially desulphurized        feed, even if that partially desulphurized feed is chemically        different from the initial feed primarily because sulphur has        been eliminated from certain compounds, and also because of        saturation of a portion of the aromatics and elimination of a        fraction of that feed because of the formation of light gaseous        products in the reaction steps which are not recovered in liquid        form;    -   the term “hydrotreatment” relating to the process of the        invention is applicable to processing a hydrocarbon feed under        hydrogen pressure, the total pressure being in the range from        about 2 to about 20 MPa, to carry out one or more chemical        reactions from the group constituted by the following reactions:        hydro-desulphurization, hydro-denitrogenation,        hydro-demetallization (to eliminate one or more metals such as        vanadium, nickel, iron, sodium, titanium, silicon, copper), and        hydrodearomatization;

PRIOR ART

Hydrotreatment processes carried out in at least two steps are alreadyknown wherein the first step is generally a desulphurization step andthe second step (or last step) is either a deep desulphurization step ora dearomatization step, or a combination of desulphurization anddearomatization; each step can comprise one or more reactors, one ormore catalytic zones (or beds), and can use identical or differentcatalysts.

The catalysts used for hydrotreatment (hydrodesulphurization and/orhydrodemetallization, and/or hydrogenation, in particular of aromatics,and/or hydrodearomatization) generally comprise a porous mineralsupport, at least one metal or metal compound from group VIII of theperiodic table (said group comprising cobalt, nickel, iron, rhodium,palladium, platinum, etc) and at least one metal or compound of a metalfrom group VIB of the periodic table (said group comprising molybdenum,tungsten, etc).

The sum of the metals or metallic compounds, expressed as the weight ofmetal with respect to the weight of finished catalyst, is usually in therange 0.5 to 45% by weight.

The sum of metals or compounds of metals from group VIII, expressed asthe weight of metal with respect to the weight of finished catalyst, isusually in the range 0.5% to 15% by weight.

The sum of metals or compounds of metals from group VIB, expressed asthe weight of metal with respect to the weight of finished catalyst, isusually in the range 2% to 30% by weight.

In a non-limiting manner, the mineral support can comprise one of thefollowing compounds: alumina, silica, zirconia, titinium oxide,magnesia, or two compounds selected from the preceding compounds, forexample silica-alumina or alumina-zirconium, or alumina-titanium oxide,or alumina-magnesia, or even three or more compounds selected from thepreceding compounds, for example silica-alumina-zirconium orsilica-alumina-magnesia.

The support can also either partially or completely comprise a zeolite.

A frequently used support is alumina, or a support composed principallyof alumina (for example 80% to 100% of alumina); said support can alsocomprise one or more other elements or promoter compounds based, forexample, on phosphorus, magnesium, boron, silicon, or comprising ahalogen. As an example, the support can comprise 0.01% to 20% by weightof B₂O₃, or SiO₂, or P₂O₅, or a halogen (for example chlorine orfluorine), or 0.01% to 20% by weight of a combination of a plurality ofthese promoters.

Examples of routinely used catalysts are catalysts based on cobalt andmolybdenum, or on nickel and molybdenum, or on nickel and tungsten, onan alumina support; said support can comprise one or more promoters suchas those cited above.

Frequently, other catalysts comprising at least one noble metal or acompound of a noble metal are used, said noble metal usually beingrhodium, palladium or platinum, and usually palladium or platinum (or amixture of said elements, for example palladium and platinum).

The quantity of noble metal or noble metals in such catalysts is usuallyin the range 0.01% to about 10% by weight with respect to the finishedcatalyst.

Such noble metal type catalysts are generally more efficient thanconventional catalysts, in particular for hydrogenation, and allow lowertemperatures to be used with lower catalytic volumes. However, they aremore expensive and more sensitive to impurities.

The operating conditions for hydrotreatment are well known to theskilled person:

The temperature is typically in the range about 200° C. to about 460° C.

The total pressure is typically in the range from about 1 MPa to about20 MPa, generally in the range 2 to 20 MPa, preferably in the range 2.5to 18 MPa, and highly preferably in the range 3 to 18 MPa.

The overall hourly space velocity of the liquid feed for each catalyticstep is typically in the range from about 0.1 to about 12, and generallyin the range from about 0.4 to about 10.

The hydrogen purity in the recycle loop is typically in the range 50 to100.

The quantity of hydrogen with respect to the liquid feed for eachcatalytic step is typically in the range from about 50 to about 1200Nm³/m³ at the reactor outlet, and usually in the range. from about 100to about 1000 Nm³/m³ at the reactor outlet.

Other elements linked to the operating conditions, to the gaspurification techniques, and to the catalysts used in hydrotreatment canbe found in published documents and patents, and in particular but notin a limiting manner to the documents or patents cited in the presentdescription, in particular in European patent application EP-A-0 1 063275, pages 5 and 6.

In order to carry out the invention, and for each hydrotreatmentreactor, the skilled person could employ one or more catalysts and theoperating conditions disclosed in the prior art documents, in particularthose summarized in the present application. The process of theinvention is not, however, bound to a particular hydrotreatment catalystor to particular operating conditions, but can be used with anyhydrotreatment catalyst (or catalysts) and any hydrotreatment operatingconditions already known to the skilled person or which could bedeveloped in the future.

United States patent U.S. Pat. No. 6,221,239 describes a concatenationof hydrotreatment with vapour stripping of the products andhydrodearomatization. Stripping is conventional steam stripping. Thepressure and the operating conditions of the stripper, in particular theoperating pressure, are not specified, and thus it can be assumed thatit is a conventional stripper for hydrotreatment products. Such a H₂Sstripper, typically disposed at the outlet from a hydrotreatment unit,operates at a relatively low pressure, conventionally about 0.5 to 1.2MPa, to encourage stripping, and avoids any condensation of water in thestripper itself. The hydrotreatment strippers also operate at arelatively low pressure to be able to use less vapour, and also low ormedium pressure vapour, which is relatively cheaper than high pressurevapour. A typical variation of the facility of said process thus has twosuccessive units: hydrotreatment with vapour stripping of H₂S, then adearomatization unit, each unit having its hydrogen recycle loop. Thatprocess employs specific catalysts for the two units, including acatalyst based on a noble metal.

That process can produce deep denitrogenation and desulphurization atthe outlet from the first reaction section, and can use a noble metalcatalyst for the hydrogenation section. It can also produce a highhydrogen purity in the hydrogenation section, vapour stripping possiblyallowing elimination of the light hydrocarbons over and above H₂S.However, it has a number of disadvantages:

The first is linked to energy efficiency, which is not optimal sincesteam is used for stripping, which steam has to be produced with aconsiderable energy output, then condensed.

Further, the catalyst comprising a noble metal or noble metal compoundis not used under ideal conditions: steam stripping results in a highwater content in the stripped liquid; however, the water content shouldbe minimized as water is typically deleterious to the activity ofhydrotreatment catalysts of that type.

The use of a low pressure stripper means that the stripped product hasto be pumped at a high differential pressure. It also necessitatesreheating the feed for the stripper from a temperature by about 50° C.to about 200° C. to 300° C. (in a series of two hydrotreatment units,the effluent from each unit is typically cooled to about 50° C.). Thisrequires the installation of a number of exchangers.

Patents concerning hydrotreatment processes with two or more stepsintegrated with intermediate H₂S stripping using high pressure hydrogenare also known; such processes mean that the second and/or last step canbe operated with a very low H₂S content:

U.S. Pat. No. 5,114,562 describes a process for hydrotreatment of amiddle distillate in at least two consecutive steps to producedesulphurized and dearomatized hydrocarbon cuts comprising a firsthydrodesulphurization step the effluent from which is sent to a hydrogenstripping zone to eliminate the hydrogen sulphide it contains. Thedesulphurized fraction obtained is sent to a second reaction zone,particularly for hydrogenation, comprising at least two reactors inseries in which the aromatic compounds are hydrogenated. The strippingzone does not have a reflux; the light hydrocarbons entrained at thestripper head and which are recycled to the hydrogenation then to thehydrodesulphurization reduce the partial pressure of hydrogen requiredfor the reaction. That process uses a single recycle loop which resultsin substantial uniformization of the hydrogen purity, in particular theamount of light hydrocarbons containing 1 to 4 carbon atoms (C1, C2, C3,C4) in the different reaction zones and in the use of fairly closeoperating pressures in said zones.

U.S. Pat. No. 5,110,444 describes a process comprising hydrotreatment ofa middle distillate in at least three distinct steps. The effluent fromthe first hydrodesulphurization step is sent to a hydrogen strippingzone to eliminate the hydrogen sulphide it contains. The desulphurizedliquid fraction obtained is sent to a first hydrogenation zone theeffluent from which is sent to a second stripping zone distinct from thefirst. Finally, the liquid portion from the second stripping zone issent to a second hydrogenation zone. The stripping zone does not includea reflux, and the light hydrocarbons entrained at the head of thestripper are also recycled to the hydrodesulphurization, which causes areduction in the partial pressure of hydrogen in that step. That processthus uses a single recycle loop, with the same consequences as thepatent cited above.

European patent application EP-A-1 063 275 describes a process forhydrotreatment of a hydrocarbon feed comprising a hydrodesulphurizationstep, a step in which the partially desulphurized effluent is sent to ahydrogen stripping zone then to a hydrotreatment step to obtain apartially dearomatized and substantially desulphurized effluent. In thatprocess, the gaseous effluent from the stripping step is cooled to atemperature sufficient to form a liquid fraction which is returned tothe head of the stripping zone. The pressure in thehydrodesulphurization and hydrotreatment steps is in the range 2 MPa to20 MPa. There is no suggestion of any advantage in operating atdifferent pressures that are higher or lower in one of the two steps.

Elsewhere, one or two recycle compressors are suggested, said twocompressors taking in the recycle gas from a common line, one of the twocompressors taking in the gas after a drying and desulphurizationoperation.

Because of their common origin (circulation in a common line), the gassupplying the two compressors have substantially identical compositionsas regards their light hydrocarbon contents (C1, C2, C3, C4). Theprocess described thus uses two recycle loops which are mixed in acommon line, which can be assimilated with a single recycle loop andalso results in uniformization of the purity of the recycle gas in thedifferent reaction zones and the use of relatively close pressures.

All of those hydrotreatment processes with intermediate pressurizedhydrogen stripping have common elements: stripping is carried out withhydrogen (typically water-free or low in water); after stripping,hydrogen is recycled in the hydrogen loop, which results in stripping athigh pressure (that of the hydrogen loop), and which does notsubstantially depressurize the liquid effluent from the first step priorto stripping. The two hydrotreatment steps are also highly integratedwith a common or mixed hydrogen loop (mixture of hydrogen from thedifferent circuits).

By comparison with the process of U.S. Pat. No. 6,221,239, thoseprocesses have several advantages: no steam is consumed for stripping;no additional water is introduced into the stripped liquid; there is noneed for a pump or only one pump with a relatively low differentialpressure to transfer the stripped liquid.

In contrast, they have the following drawbacks:

The integrated hydrogen loop results in the use of similar pressures forthe two (or more) reaction steps, and prevents independent optimizationof the operating pressures as a function of the required specificationsfor the final product. To modify existing units (the operating pressureof which cannot be modified), the addition of a reaction step and asupplemental reactor to obtain a product with more severe specificationsmeans that this step and the reactor must be designed and sized to havea pressure close to that of the existing unit, which is sometimes farfrom optimal.

Further, when using a catalyst in step a3) that is sensitive toimpurities, the integrated or mixed recycle loop results either inprocessing the hydrogen of that loop using expensive means to eliminateimpurities, or using a cheaper treatment and accepting traces ofimpurities and conditions of service for the catalyst that are notoptimal.

Finally, the integrated or mixed recycle loop leads to the presence oflight hydrocarbons in the recycle gas in all of the reaction steps,while said hydrocarbons are only generally produced in large quantitiesduring the first desulphurization step. This results in a reduction inthe purity of the hydrogen and the partial pressure of hydrogen(molecular), which reduces the rate of the hydrotreatment reactions.

The technologies used in the process with steam stripping and in theintegrated processes with intermediate stripping with pressurizedhydrogen are not equivalent and cannot be combined: it is not possibleto carry out simultaneous hydrogen and steam stripping under high andlow pressure, nor is it possible to use both a non-integrated processand a highly integrated process at the same time.

However, the Applicants have surprisingly discovered that it is possibleto carry out a process having the advantages of the processes describedabove without suffering their drawbacks. The process of the inventioncan in particular:

-   -   independently optimize the operating pressures of the different        reaction steps;    -   provide high energetic efficiency, not necessitating the        consumption of steam for the intermediate stripping between the        reaction steps, typically with limited cooling and/or heating of        the product between the two reaction steps (upstream/downstream        of the stripping column);    -   carry out stripping without notable depressurization and without        requiring a pump to take up the product stripped at a high        differential pressure;    -   obtain a recycle gas that is substantially free of impurities        such as H₂S and/or H₂O in the second reaction step, and thus can        use high performance impurity-sensitive catalysts under optimal        conditions without very expensive treatment of all of the        hydrogen supplying the step under consideration;    -   greatly reduce the quantity of light hydrocarbons present in the        recycle gas supplying the second reaction step, which increases        the purity of said gas and the catalytic activity.

DESCRIPTION

A typical feed for the process of the invention is a middle distillatefeed. Within the context of the present invention, the term “middledistillate” designates hydrocarbon fractions boiling in a range of about130° C. to about 410° C., generally about 140° C. to about 375° C., forexample about 150° C. to about 370° C. A middle distillate feed can alsoinclude a gas oil or diesel cut or can be designated by one of theseterms.

The process of the present invention can also be applied to thetreatment of straight run hydrocarbon fractions with a boiling pointlocated in the naphtha range: it can be used to produce hydrocarbon cutsfor use as solvents or diluents and preferably containing a reducedaromatics content; the term “naphtha” designates a hydrocarbon fractioncomprising hydrocarbons containing 5 carbon atoms to hydrocarbons withan end point of about 210° C.

The process can also be used for the hydrotreatment and desulphurizationof gasoline, in particular gasoline produced by a fluid catalyticcracking facility (FCC) or other gasoline fractions deriving, forexample, from cokefaction, visbreaking, or residue hydroconversionunits, the term “gasoline” designating a hydrocarbon fraction from acracking unit boiling between about 30° C. and about 210° C.

A further possible feed is kerosene. The term “kerosene” designates ahydrocarbon fraction boiling in the range about 130° C. to 250° C.

The process of the invention can also be used for hydrotreating heaviercuts, such as a vacuum distillate boiling in the range about 370° C. to565° C.

The process of the invention can also be used for hydrotreating heaviercuts than a vacuum distillate, in particular deasphalted oil cuts.

The term “deasphalted oil” designates a cut boiling above about 565° C.(or at a slightly lower temperature such as about 525° C.) obtained bydeasphalting a heavy residue, for example a vacuum residue, using apropane, butane, pentane, light gasoline type solvent or any othersuitable solvent that is known to the skilled person.

Finally, the process can be used for hydrotreating a broader hydrocarboncut resulting, for example (in a non limiting manner,) from mixing atleast two of the fractions defined above.

It is also possible to use residual feeds, for example the vacuumresidue boiling above about 565° C. and comprising asphaltenes,non-vaporizable.

The process of the invention comprises the following steps:

-   -   a first step a1) for hydrotreatment in which said feed and        excess hydrogen are passed over a first hydrotreatment catalyst        to convert at least the major portion of the sulphur contained        in the feed into H₂S;    -   downstream of step a1), a step a2) for stripping the at least        partially desulphurized feed from step a1) in a pressurized        stripping column, preferably with a liquid reflux, using at        least one hydrogen-rich stripping gas, to produce at least one        hydrogen-rich gaseous stripping effluent and at least one liquid        stripping effluent, the gaseous stripping effluent being at        least partially compressed and recycled to the inlet to the        first step a1) using a first recycle loop REC1;    -   downstream of step a2), a second hydrotreatment step a3) in        which the stripped liquid effluent and excess hydrogen are        passed over a second hydrotreatment catalyst (different from or        identical to the first), the effluent from said step a3) being        fractionated, in a gas/liquid separation step a4), into a        hydrogen-rich gaseous fraction and a hydrotreated liquid        fraction, said hydrogen-rich gaseous fraction being at least        partially compressed and recycled to the inlet to step a3) using        a second recycle loop REC2 separate from the loop REC1.

The term “separate loop” means that the two loops are distinct (comparedwith a single loop supplying the different hydrotreatment steps inparallel or in series), they comprise different means for compressingthe recycle gas, and have neither a common portion nor a common pointwhere the recycle gas supplying the two loops is collected and mixed; incontrast, it does not exclude connections between the two loops such asone or more lines for evacuating a purge gas from one loop to the otherloop.

If the purge gas is evacuated from loop REC2 to loop REC1, there is noneed to treat the purge gas the impurities content of which, inparticular H₂S, is low, while the hydrotreatment step a1) is oftencarried out with a substantial H₂S content.

If the purge gas is evacuated from loop REC1 to loop REC2, it ispreferable for that purge gas does not introduce too high a proportionof undesirable impurities into loop REC2. Thus, that purge gas couldgenerally be treated (alone or possibly mixed with a further recycle gasstream) to eliminate the major portion of the H₂S contained in the purgegas, and possibly water if the catalyst for the second step is highlysensitive to water. Examples of such treatments are given below in thepresent application.

The flow rate of the purge gas is generally relatively low compared withthe flow rate of a recycle loop, and so treatment of any purge gas (fromloop REC1 to loop REC2) is only applied at a low gas flow rate(typically less than 50% or even less than 30% of the flow rate of eachof the recycle loops REC1 and REC2 (flow rate measured by convention atthe recycle compressor).

In a preferred embodiment, the loop REC2 is supplied with hydrogenindependently of the loop REC1 by one or more streams of hydrogenexclusively constituted by stream(s) of external makeup hydrogen to loopREC1. This produces a recycle gas with very high purity in loop REC2.

In the process of the invention, loop REC2 is not common, has no commonpart and has no mixing point with loop REC1 combining the two recyclegases. In the preferred implementation in which loop REC2 is suppliedwith hydrogen independently of loop REC1, it is thus not polluted withcompounds that may be found in loop REC1.

The absence of a mixing point also means that the pressures in the twoloops can be decoupled and the pressure of the second loop in particularcan be adapted to its requirements. This has an advantage whensatisfying severe specifications, which in some cases, for example withvery low quality feeds, can require relatively high pressures in one ofthe hydrotreatment steps (especially the last step), but not necessarilyboth steps. As an example, a pressure. in step a3) that is higher thanthat in step a1) by at least 1.2 MPa can be used.

In other cases, however, the pressure in the second step can berelatively close to that in the first step, for example a pressure thatis higher than or equal to the first step by about 0 to 1.2 MPa, or evena pressure lower than that of the first step.

Decoupling of the two loops in combination with stripping can alsoeliminate from loop REC2 pollutants which are only present in largequantities in part of the recycle (loop REC1) and to use highperformance catalysts which are sensitive to certain pollutants in thesecond or last hydrotreatment step without reintroducing thosepollutants in the second or last hydrotreatment step via a recycle gasthat is common or mixed for the two steps. In the case in which purgegas is evacuated from the first loop REC1 to the second loop REC2, thecost of the treatment for purifying said purge gas (H₂S elimination andpossibly water elimination) is limited, much lower than the cost oftreating all of the recycle gas for the REC2 loop because the flow rateof the purge gas is typically relatively low (for example at most 50%and in particular at most 30% of the flow rate of the gas in any one ofthe two loops).

The stripping column, which preferably operates at a (column head)pressure close to the pressure of step a1) (for example at a pressurelower than that in step a1) by about 0 to 1 MPa, preferably about 0 to0.6 MPa), can essentially eliminate compounds that are undesirable forthe second reaction step: using a hydrogen-rich stripping gas that issubstantially free of pollutants: H₂S, NH₃ (and possibly water), thesepollutants can be stripped and essentially eliminated from the columnbottom product (the liquid stripping effluent) before supplying thesecond reaction step.

The degree of hydrodesulphurization in step a1) and the strippingefficiency (linked to the number of theoretical plates in the column)are preferably adjusted so that the H₂S content in the loop REC2 islimited, for example to less than 1000 ppm or 200 ppm. A preferredcontent, in particular if a thioresitant catalyst is not used, isgenerally less than 100 ppm, or even less than 50 ppm. The mostpreferred content is below 10 ppm, in particular less than 5 ppm. In thecase in which a sulphur-resistant catalyst is used in the secondreaction step, however, it is possible to have more than 500 ppm or evenmore than 1000 ppm of H₂S in the REC2 loop.

The stripping zone of the stripping column (the zone located below thesupply) can, for example, have an efficiency corresponding to 3 to 60theoretical plates, and generally 5 to 30 theoretical plates, forexample 8 to 20 theoretical plates, limits included.

A preferred implementation of stripping step a2) consists of using astripping column that also includes a zone for rectification of thestripping vapours (located above the supply) with a liquid reflux(substantially to the head of the rectification zone). Rectificationensures that substantially all of the liquid products flow back to thecolumn bottom or possibly principally the relatively heavysulphur-containing products that are difficult to desulphurize,depending on the variation of the process of the invention, as explainedbelow. It can recover and subject to the second hydrotreatment step allcompounds for which deep complementary hydrotreatment is sought.

The number of theoretical plates in the rectification zone is generallyin the range 1 to 30, preferably in the range 2 to 20, and highlypreferably in the range 5 to 14, limits included.

Preferably, the total effluent from the reactor for step a1) suppliesthe stripping column, the gas contained in the effluent from the reactoralso undergoing rectification using a liquid reflux. However, the scopeof the invention also encompasses prior separation of the gas containedin the effluent from the step a1) reactor upstream of the strippingcolumn.

Preferably, the effluent from reaction step a1) is only partially cooledbefore entering the stripping column. The inlet temperature for thestripping column is generally at least 140° C., often at least 180° C.,and frequently in the range 180° C. to 390° C.

Preferably, the liquid reflux is obtained by cooling and partiallycondensing the column head vapours then separating the cooled stream ina gas/liquid separator drum or a reflux separator drum. Preferably, saidvapours are cooled to a temperature of 80° C. or less, for example about50° C. or lower. The cooled gas from said reflux drum then constitutes a“gaseous stripping effluent” which can optionally be treated then istypically compressed and recycled.

In a first variation of the process of the invention, substantially allof the relatively light hydrocarbon liquid phase is returned to thestripping column as an internal reflux, and thus no light liquidstripping liquid effluent is produced at the reflux drum, or possiblyonly a very small quantity generally less than 10% by weight of theinitial feed, for example a reduced quantity of naphtha or other lightproducts that are often generated during the first step a1).

In said first variation, typically the quantity of liquid strippingeffluent is sought to be maximized, recovered from the column bottom andcompounds boiling in the desired distillation interval are preventedfrom leaving the column head either in the form of a light liquidstripping effluent or with the gaseous stripping effluent. Thus,preferably, a relatively low temperature is used in the reflux drum.

The liquid stripping effluent in this first variation thus preferablyrepresents at least 90% by weight and usually about 95% by weight ormore of the initial feed.

The aim of this first variation is to subject the largest possiblequantity of product boiling in the desired distillation interval to thesecond hydrotreatment step a3), the main aim usually being to carry outmajor hydrodearomatization at said second step, applied to the largestpossible portion of the treated cut, typically to increase the cetaneindex by a maximum amount. Step a3), however, also carries out moresevere desulphurization of the feed. To carry out this main objective ofdeep hydrogenation in the second reaction step, then, one or more highlyeffective hydrogenation catalysts are generally used in step a3), inparticular and preferably a catalyst of the noble metal type, forexample a platinum on alumina or platinum/palladium on alumina type.

In the first case (platinum catalyst in step a3)), highly severedesulphurization is preferably carried out in step a1), for example toabout 100 ppm of sulphur or, as is preferable, about 50 ppm, for exampleabout 10 ppm or less, to limit the quantity of residual sulphur in stepa3) because of the high sensitivity of the catalyst to sulphur. As anexample, it is possible to use a nickel/molybdenum on alumina typecatalyst in step a1). The flowchart for the process of the invention,with loop REC2 separate from loop REC1, can also almost eliminate waterat the stripping column (with stripping preferably using makeup hydrogentypically with a substantially zero water content) without reintroducingwater to the loop REC2. A very low water content (for example less thanabout 200 ppm, often less than 100 pm and generally less than 10 ppm) isreadily obtained, which is favourable to the activity of the platinumcatalyst. When using a purge gas from loop REC1 to loop REC2, it isoften preferable with a platinum catalyst to eliminate H₂S from thepurge gas and to dehydrate it before introducing it into the REC2 loop.

In the second case (platinum/palladium catalyst in step a3)), less deepdesulphurization is optionally carried out in step a1), for example toabout 1000 ppm of sulphur or, as is preferable, about 500 ppm, forexample to about 100 ppm of sulphur or to a content compatible with goodefficiency of the platinum/palladium catalyst used. as an example, it ispossible to use a cobalt/molybdenum on alumina type catalyst in stepa1). A higher water content than in the preceding case is acceptable instep a3), but in practice almost complete water elimination can readilybe achieved by stripping with makeup hydrogen. When using a purge gasfrom loop REC1 to loop REC2, and a platinum/palladium catalyst in stepa3), it is often preferable to eliminate H₂S from that purge gas and/orto dehydrate it before introducing it into the REC2 loop.

Finally, a conventional catalyst (for example of the nickel/molybdenumon alumina type), which is less sensitive to impurities but of lowerperformance, could be used in step a3).

The supply temperature for the stripping column that can be used in thisfirst variation is typically in the range from about 140° C. to about270° C., preferably in the range 180° C. to 250° C.

Two typical cases for the operation of the first variation of theprocess of the invention are given in the Examples (1 and 2).

In a second variation of the process of the invention, which can be usedto carry out deep desulphurization of middle distillates, in contrast,the production of a light stripping effluent in large quantities issought (typically between 10% and 70%, in particular between 20% and 60%by weight with respect to the initial feed), which is evacuated directlydownstream (i.e. without complementary hydrotreatment). The designparameters and operating conditions are selected so that this lightliquid stripping effluent and the liquid reflux, which typically haveidentical compositions, constitute a product with a very low (organic)sulphur content at the required specifications (less than 50 ppm, oftenless than 30 ppm, or even less than 10 ppm, for example about 5 ppm ofsulphur). The degree of desulphurization in the first reaction step a1)must in particular be adapted to this very low desired sulphur contentfor the light liquid stripping effluent. Usually, a 95% distillationpoint for the light liquid stripping effluent is preferably selected tobe 200° C. to 315° C., more preferably between 235° C. and 312° C. tosubstantially avoid the presence of heavy fractions in the feed, whichin many cases comprises sulphur-containing products that are relativelyrefractory to desulphurization, for example dibenzothiophenes which arefed back to the column bottom.

In this second variation, the second reaction step usually has theprincipal aim of deep desulphurization of the heavy fractions in thefeed. Typically, the liquid stripping effluent (feed for step a3)) has aconsiderable sulphur content, for example in the range 50 to 2000 ppm,and normally between 100 and 1000 ppm, usually between 100 and about 500ppm. The principal importance of the production and evacuation of alight stripping effluent in large quantities in this second variation isthat this substantially reduces the flow rate of the feed to the secondreaction step a3), and thus the required volume for the reactor for saidstep.

The most appropriate catalysts for step a3) in this second variation ofthe process of the invention are catalysts adapted for deepdesulphurization, in particular platinum/palladium on alumina typecatalysts. It is also possible to use conventional catalysts, forexample of the nickel/molybdenum on alumina type, or other catalyststhat can carry out final desulphurization (at the same time in generalas a certain amount of dearomatization of the heavy fraction of thefeed, supplied to step a3)).

In this second variation, in the case in which a purge gas is used fromloop REC1 to loop REC2, it is also possible to carry out a treatment toeliminate H₂S and/or dehydrate the purge gas before supplying the loopREC2.

The supply temperatures for the stripping column for use in this secondvariation are typically in the range from about 220° C. to about 390°C., preferably in the range 270° C. to 390° C., and highly preferably inthe range 305° C. to 390° C., for example in the range about 315° C. to380° C. Preferably, a supply temperature for the stripping column isused which may differ by at most 90° C., usually at most 70° C. from theoutlet temperature from the first reaction step a1). Generally, thestripping column is supplied after limited cooling (by at most 90° C.,usually by at most 70° C.) or without cooling the effluent from thereactor for step a1).

This variation of the process of the invention forms the subject matterof a separate patent application made simultaneously with the presentapplication.

The two variations described above are not limiting and the process ofthe invention can also be used in other variations and/or for otherhydrotreatment objectives and/or catalysts and/or operating conditions.In particular, the catalyst (or catalysts) used in step a1) are notlimited to conventional cobalt/molybdenum or nickel/molybdenum onalumina types; the catalyst (or catalysts) used in step a3) are notlimited to platinum on alumina or platinum/palladium on alumina ornickel/molybdenum on alumina type catalysts. The process of theinvention can be used with any type of hydrotreatment catalyst.

The operating conditions that are suitable for each step, in particularfor deep desulphurization in step a1), can be severe: a low hourly spacevelocity (HSV), a high temperature, a high partial pressure of hydrogen,a catalyst suitable for the feed and for severe conditions. Adequateoperating conditions and the choice of an adequate catalyst will dependgreatly on the feed that is treated but can readily be determined for agiven feed by the skilled person.

A variety of modes can be used to carry out the variations in theprocess:

As an example, it is also possible to recycle to the inlet to thestripping column (to control the inlet temperature) a further portion ofthe relatively light hydrocarbon liquid phase recovered from the refluxdrum. The scope of the invention also encompasses recycling a portion ofthe relatively light hydrocarbon liquid phase to the inlet to step a1).

For the second variation of the process, it is also possible to cool thecolumn head vapours in two steps:

-   -   an initial cooling with partial condensation of the column head        vapours followed by gas/liquid separation in a reflux drum and,        for example, returning all of the condensed liquid to the        column, the temperature in this drum being in the range 70° C.        to 250° C., for example, and such that a large quantity of the        hydrocarbons remains in the gas from said separator drum;    -   complementary cooling of the gas from said latter separator drum        with or without contact with a liquid (for example the        hydrotreated liquid fraction) that is capable of absorbing light        hydrocarbons, to condense and evacuate the light liquid        stripping effluent alone or as a mixture.

The flow rates for the liquid reflux and stripping gas depend stronglyon a number of parameters including the temperature of the supply to thestripping column, for example. Preferably, said parameters are selectedin a coordinated manner. Generally, a stripping gas flow rate in therange 2.5 to 520 Nm³/m³ of feed supplied to step a1) is used, usually inthe range 5 to 250 Nm³/m³ of feed supplied to step a1). Preferably, thisflow rate corresponds to a hydrogen flow rate in the range 5% to 150%,preferably 10% to 100% of the flow rate of the hydrogen consumed in stepa1) (assuming all of the stripping gas is consumed). The quantity ofliquid reflux is generally in the range 0.05 to 1.2 kg/g of liquid feedsupplied to step a1), and usually in the range 0.15 to 0.6 kg/g ofliquid feed supplied to step a1). Suitable stripping gas and liquidreflux flow rates can readily be determined by the skilled person forthe desired separation conditions by computer simulation of thefractionation.

In an optional preferred disposition of the process of the invention,washing water is injected into the vapours at the stripping column head,upstream of the cooling exchanger or exchangers (and/or an air-cooledexchanger) to capture nitrogen-containing compounds, for example ammoniaand ammonium sulphide formed in the reactor; the aqueous phase,containing a large portion of these undesirable compounds, is preferablyrecovered downstream in a gas/liquid separator drum also functioning asa decanter, then evacuated.

The process of the invention can also be carried out with a variety ofmodifications and in a variety of implementations.

In a highly preferable variation of the process which produces a lightliquid stripping effluent, a substantially desulphurized light productis produced. Optionally, if the light liquid stripping effluent does notcompletely satisfy the required specifications, it is possible for it toundergo complementary less severe hydrotreatment to bring it to therequired specifications.

In one implementation of the process, each of loops REC1 and REC2 issupplied with makeup hydrogen in a quantity adapted to the hydrogenconsumption in said loop. The two loops can then operate without a purgegas.

In a further implementation of the process of the invention, an excessof makeup hydrogen with respect to the hydrogen requirement in step a3)is supplied to step a3) at at least one point in the loop REC2, and ahydrogen-rich purge gas flow (corresponding to the excess hydrogen) isextracted from the loop REC1. Advantageously, all or part of the purgegas can be used as a stripping gas in step a2): this purge gas, which issubstantially free of impurities as it derives from loop REC2, providesan additional stripping gas which can advantageously be introduced intothe column at an intermediate position, below the inlet for the effluentfrom step a1), and above the (optional) inlet for the higher purityexternal stripping gas (makeup hydrogen).

This purge for the loop REC2 can, in this implementation of the processof the invention, represent 10% to 100% , in particular 30% to 100% ofthe hydrogen requirements for the loop REC1. In the case in which saidpurge represents 100% of the hydrogen requirements for the loop REC1,the loop REC1 is only supplied with makeup hydrogen via the purge forthe loop REC2.

Finally, in a further implementation, loop REC1 can be supplied withexcess makeup hydrogen and the purge gas can be evacuated to the loopREC2, preferably after eliminating H₂S and usually after dehydration.

In general, the process of the invention can advantageously comprise astep a5) for contact (or contacting) at least a portion of thehydrotreated liquid fraction from step a4) with at least a portion ofthe gas stream moving in the loop REC1. The effluent from said step a5)is then separated into a liquid contacting effluent and a gaseouscontacting effluent; at least a portion of this gaseous contactingeffluent is then recycled to the loop REC1.

Contact allows the liquid from the second hydrotreatment reaction step(step a3)) to be used after gas/liquid separation as an adsorbent forlight hydrocarbons containing 1 to 4 carbon atoms (C1, C2, C3, C4)contained in the gas from the loop REC1, and to increase the hydrogenpurity of said loop REC1: step a3), the supply for which has beenstripped, and which produces relatively few light compounds, operateswith a high hydrogen purity, and the liquid from this step constitutes agood adsorbent for light hydrocarbons.

The process can also comprise a treatment for eliminating at least partof the H₂S contained in at least part of the gaseous stream moving inthe loop REC1, said treatment being carried out at a point in the loopREC1 located generally downstream of the stripping step a2) and upstreamof the contact step a5). Said treatment can consist of gas washing withan amine solution, a technique that is well known to the skilled person,or H₂S elimination using another process that is known to the skilledperson. Such treatments are, for example, shown in EP-A-1 063 275.

It is also possible with the process of the invention in some cases (forexample when the sulphur content in the feed is not too high or if arelatively low hourly space velocity is used in step a1), which cancompensate for a high sulphur content in the recycle gas to the loopREC1), not to use a H₂S elimination treatment by amine washing therecycle gases moving in the facility. In this case, the process will,however, comprise one or more treatments to eliminate at least part ofthe H₂S contained in the recycle gases moving in loops REC1 and REC2, inwhich each of the treatment or treatments is constituted by acombination of a step for contact of at least a portion of the recyclegas moving in the loop REC1 with a hydrocarbon liquid fraction, to carryout limited H₂S absorption by this liquid hydrocarbon fraction, followedby a step for gas/liquid separation of the mixture deriving from thiscontact and direct evacuation of at least a portion of the separatedliquid (downstream of the process, i.e. with no complementaryhydrotreatment). The liquid hydrocarbon fraction which can absorb (andevacuate) H₂S can optionally be a portion of the relatively lighthydrocarbon liquid phase (typically recovered at the reflux drum) and/orusually all or part of the hydrotreated liquid fraction. The term “(H₂Selimination) treatment” as used in this disposition of the process mustbe taken in its general sense, thus comprising simple contact with aliquid phase to absorb H₂S by liquid/vapour equilibrium, and not onlychemical or physico-chemical treatments such as amine washing. Contactcan also be achieved by partial condensation of a hydrocarbon liquidphase and not solely by contacting with all or part of the hydrotreatedliquid fraction. In other words, this (optional) disposition of theprocess means that it is possible to use hydrotreatment in two or moresteps with intermediate pressurized hydrogen stripping, which canoptionally include the use of a noble metal catalyst (for example of theplatinum or platinum/palladium on alumina type) in the second and/orlast step, without amine washing of a portion of the recycle gas.Typically, neither the recycle gas nor the hydrogen makeup or makeups istreated by amine washing in this optional disposition of the process ofthe invention.

The H₂S contained in the gas from the REC1 loop (produced in step a1))is then evacuated by contact of said H₂S-rich gas with the liquidhydrocarbon fraction, to absorb H₂S which is then evacuated with theliquid product to the stripper downstream of the process. Thisabsorption with an internal hydrocarbon liquid is less efficient thanamine washing and results in a higher amount of H₂S in the recycle gasfor the loop REC1. in contrast, it avoids the use of an expensivecircuit for circulating and regenerating an amine solution.

The loop REC2, which is separate from REC1 in the process of theinvention, means that a H₂S content which remains relatively high inloop REC1 in the absence of any amine washing is not reflected to loopREC2. Thus, a noble metal type catalyst can readily be used in step a3).In contrast, the processes of the prior art with two hydrotreatmentsteps and an integrated hydrogen stripper all require amine washing.

The loop REC1 often contains water because of an injection of washingwater and/or the presence of water in the feed. Because makeup hydrogen,which is generally substantially free of water, is preferentially (andoften exclusively) used as a stripping gas (and/or if not, a gas with avery low water content, for example less than 5 ppm), then typicallyvery good water elimination of the stripping liquid effluent isobtained. In the case in which a purge is used from loop REC1 to loopREC2, it is preferable to eliminate H₂S from this purge gas, but alsousually to dehydrate it, using techniques that are known to the skilledperson, for example on a molecular sieve or other solid adsorbent, or bywashing with a dessicant liquid, for example diethyleneglycol ortriethylene glycol, or any other known dehydration process. The need forsaid dehydration and the desired degree of dehydration essentiallydepend on the sensitivity to water of the catalyst for step a3)(typically high for platinum catalysts, medium for platinum/palladiumcatalysts, and low for conventional catalysts with no noble metal).Suitable conditions for (optional) dehydration can readily be determinedby the skilled person.

A further aim of the stripping step, apart from at least partialelimination of many impurities, is to remove by stripping a substantialportion of the light hydrocarbons containing 1 to 4 carbon atoms (C1 toC4) present in the liquid effluent from step a1), prior to step a3).This can produce a purity Pur2 of hydrogen in loop REC2 that is greaterthan the purity Pur1 of the hydrogen in the loop REC1. As an example, itmay be possible to have a Pur2/Pur1 ratio of more than 1.06, inparticular more than 1.08 and usually more than 1.10.

By way of non limiting example, the following ranges of purity can beobtained:

With a makeup hydrogen purity of 92, it is generally possible to obtaina purity in the range about 58 to 84, and often in the range about 60 to80 in the loop REC1, while the hydrogen purity in loop REC2 cangenerally be in the range about 73 to 90, and often in the range about75 to 88.

With a makeup hydrogen purity of 99.9, it is generally possible toobtain a purity in the range about 73 to 90, and usually in the range 75to 88 in the loop REC1, while the purity of the hydrogen in the loopREC2 can generally be in the range about 88 to 99.5, and usually in therange 90 to 99.

In a further variation of the process of the invention, separate makeuphydrogen streams (external to the two loops) with different purities aresupplied to the two loops REC1 and REC2. Preferably, loop REC2 issupplied with a stream of makeup hydrogen with higher purity than thatof the stream of makeup hydrogen supplying the loop REC1. As an example,the purity of the makeup hydrogen for the loop REC1 is about 92 or inthe range 88 to 96, and derives at least in part from a catalyticreforming unit, and the purity of the hydrogen makeup for loop REC2 is99.9 or more and derives from a steam reforming unit followed by a stepfor separation (purification) on a molecular sieve or on a further bedof solid adsorbent, known to the skilled person as PSA (pressure swingadsorption). The differences in purity can be lower if the makeup usedis one or two mixtures of gas with different purities, for example ofthe type cited above, with different percentages for each of themakeups.

A further technical advantage of the process of the invention is to useloops and reaction steps that are optionally at very differentpressures, which means that the pressure can be adjusted, in particularin the second hydrotreatment step a3), to the optimum desired level.

This is interesting both for new units and for revamping existing units,for example by adding a supplementary reaction step a3).

It is possible to use a pressure in step a3) that is higher by at least1.2 MPa and at most 12 MPa than that in step a1), in particular higherby at least 1.2 MPa and at most 4.5 MPa than that in step a1).

Regarding the partial pressures of hydrogen in the two loops (byconvention at the outlet from the reactor in both cases), it is possibleto use a partial pressure of hydrogen in step a3) that is greater by atleast 1.35 MPa and at most 13.5 MPa than that of step a1), in particulargreater by at least 1.35 MPa and at most 5 MPa than that of step a1).

However, it is also possible to use closer partial pressures ofhydrogen, for example a partial pressure of hydrogen in step a3) greaterthan that in step a1) by a value in the range 0 to 1.35 MPa, inparticular in the range 0.1 to 1.35 MPa.

It is also possible to use a pressure in step a3) that is substantiallyequal to or lower by at least 0.1 MPa than that in step a1). In such acase, it is nevertheless often possible to use a partial pressure ofhydrogen in step a3) that is higher, for example by at least 0.1 MPa,than that in step a1), because of the greater purity of the loop REC2.

The invention also concerns any hydrocarbon cut from the group formed bygas, jet fuel, kerosene, diesel fuel, gas oil, vacuum distillate, anddeasphalted oil containing at least one fraction hydrotreated using theprocess of the invention.

Two implementations among the preferred implementations of the processof the invention are shown in FIGS. 1 and 2.

FIG. 1 shows a flow chart for a hydrotreatment facility for carrying outa first variation of the process of the invention:

The hydrotreatment facility feed, for example a straight run middledistillate type cut containing a high proportion or even 100% of lightcycle oil (LCO), is supplied via a line 1 and supplemented with ahydrogen-rich recycle gaseous stream moving in line 23. The mixtureformed moves in line 2 and is reheated in the feed/effluent heatexchanger 3 (and often in a heat exchanger with the effluent from stepa3), this exchanger not being shown), then sent via line 4 to a furnace5 in which its temperature is heated to the required temperature for thefirst reaction step a1). At the outlet from furnace 5, the reactionmixture moves in line 6 then supplies the hydrotreatment reactor 7 whichis typically a fixed catalytic bed downflow reactor. The effluent fromsaid reactor 7 (carrying out first reaction step a1)) is then sent via aline 8 to the feed/effluent exchanger 3 then supplied to a strippingcolumn 10 via a line 9.

This column is also supplied with two sources of hydrogen-rich strippinggas which are supplied via lines 34 and 58: the gas supplied via line 34is typically makeup hydrogen, of medium or possibly high purity, andpreferably substantially free of impurities such as H₂S and/or watervapour. This gas can, for example, derive from a catalytic reformingunit and/or a steam reforming unit.

The second stream of stripping gas supplied via a line 58 is optional.Said stream derives from a possible excess of hydrogen-rich gas (makeup)supplied to step a3); the excess can then (in particular) be used as thestripping gas supplied via line 58. This use of excess makeup hydrogenin reaction step a3) increases the purity of the recycle gas in thisstep.

The stripping column can also be supplied from other sources ofstripping gas, not shown in FIG. 1; in particular, in some casesstripping gas (if sufficiently pure) taken from the recycle gas movingin line 23 could be supplied, being introduced into the column at orjust above the supply of the purge gas for the REC2 loop via line 58.

In general, the stripping gas or gases supplying the column 10 maypreviously have been dried in a dryer (optional, not shown) tosubstantially eliminate water from the stripping step (more particularlyif the catalyst from hydrotreatment step a3) contains a noble metalwhich is highly sensitive to water).

In general, the stripping gas or gases supplying the column 10 may alsohave been purified to eliminate any traces of H₂S, for example byadsorption on a zinc oxide bed (optional, not shown), to more completelyeliminate H₂S in the stripping step (more particularly if the catalystfor hydrotreatment step a3) contains a noble metal which is highlysensitive to H₂S, for example a platinum catalyst).

This purification of the stripping gas is, however, generally notnecessary if the stripping gas is makeup hydrogen deriving from acatalytic reformer. In the case in which the makeup hydrogen is at leastpartially produced by steam reforming, then preferably, after steamreforming, almost complete elimination of compounds other than hydrogenis preferably carried out on a molecular sieve (PSA type separation)which produces very high purity hydrogen.

The vapours from the head of column 10 move in line 11 and aresupplemented with washing water supplied via a line 25, and are thencooled with partial condensation in the air-cooled exchanger 12 thentransferred via a line 13, prior to being separated in the gas/strippingstep liquid separator drum 14, which also acts as a decanter and areflux drum. Said drum 14 carries out separation between three phases:

-   -   a gas stream or “gaseous stripping step effluent” sent to line        16;    -   a relatively light hydrocarbon liquid phase extracted via line        15. A first portion of said liquid phase is recycled to the        column 10 as a liquid reflux, still via line 15; the (optional)        residual liquid fraction or “light liquid stripping effluent” is        evacuated downstream via a line 27 (optional), preferably        downstream (i.e. it is not treated in reaction step a3));    -   an aqueous liquid phase also containing nitrogen-containing        impurities, evacuated via line 26.

The liquid at the column bottom 10 or “stripped liquid effluent” (or“heavy stripped liquid effluent” if there is a light liquid strippingeffluent) is sent to the second reaction step a3) via a line 41.

The gaseous stripping effluent moving in line 16 optionally traversesthe equipment 17 to at least partially eliminate the H₂S contained inthat gas. That equipment 17 can typically be a washer, or H₂S absorberusing a solution of amines (inlet and outlet for the amine solutions arenot shown); it can also be a further device for eliminating H₂S and/or adevice for eliminating H₂S and water. That equipment 17 is optional (itsuse depends on a number of parameters, in particular on the sulphurcontent in the feed, and on the space velocity used in the first reactor7, which if it is sufficiently low can allow the desireddesulphurization in the first step a1) without amine washing in the REC1loop).

At the outlet from said equipment 17, the gas moves in a line 18 and issupplemented with a stream of “hydrotreated liquid fraction” moving in aline 59, then rejoins the gas/liquid separator drum 20 via line 19 inwhich in-line mixing occurs. This can absorb light hydrocarbons into theliquid phase and increases the purity of the recycle loop REC1.

The liquid effluent from drum 20 or “liquid contacting effluent” isevacuated via a line 24 and constitutes a liquid effluent from thehydrotreatment facility (a further (optional) effluent, the “lightliquid stripping effluent”, is optionally evacuated via line 27).

The gas separated in drum 20 or “gaseous contacting effluent” is sentvia a line 21 to a compressor 22 for the recycle gas, then recycled tothe inlet to reaction step a1) via line 23.

The stripped liquid effluent moving in line 41 is pumped through thepump 40 to bring its pressure to a sufficient value for reaction stepa3), said pressure in this example being higher than that in step a1).At the discharge from pump 40, the liquid is supplemented withhydrogen-rich gas supplied via a line 56 then moves in a line 43,traversing the feed/effluent exchanger 44, then moving in a line 45 andis heated (again) in the exchanger (or furnace) 46, then rejoins thereactor 48 for reaction step a3) via a line 47. At the outlet from saidreactor, the effluent transits via a line 49, traverses exchanger 44,moves in line 50, is cooled in air-cooled exchanger 51 then moves in aline 52 to reach the gas/liquid separator drum 53, the liquid fraction(hydrotreated) is sent to line 59 for mixing with the gas moving in line18 of the loop REC1, and the gaseous fraction is sent via a line 54 tothe gas recycle compressor 55. Optionally, a portion of said gaseousfraction (possible excess gas, i.e., any purge gas in the hydrogenrecycle loop for step a3)) is removed and sent via the (optional) line58 to the lower portion of the stripping column 10 (and/or optionally toa further point in the loop REC1 via means that are not shown).

A makeup hydrogen stream supplied via line 33 is then added to theresidual gas stream moving in line 54 upstream of the compressor 55. Therecycle gas, after said makeup hydrogen addition, is then compressed inthe compressor 55 and recycled to the inlet to the reaction step a3) viaa line 56.

This makeup hydrogen can be constituted by hydrogen deriving from acatalytic reforming unit and/or steam reforming unit (usually naphtha ora natural gas). Optionally, hydrogen with a purity that is higher thanthat of the hydrogen supplied via line 34 can be supplied via line 33.This high purity hydrogen can derive from a PSA type separation unitwhich may deliver hydrogen with a purity that is usually greater than99.9. This can increase the purity and partial pressure of the hydrogenin the loop REC2.

In the facility of FIG. 1, the recycle loop REC1 of step a1) comprisesthe elements referred to hereinafter, following the “gas path”: 21, 22,23, 2, 3, 4, 5, 6, 7, 8, 3, 9, 10 (upper portion of the column locatedabove the supply 9, the supplied gas rising in the column), 11, 12, 13,14, 16, 17, 18, 19, 20, and 21) again, which closes the loop.

The recycle loop REC2 of step a3) comprises the following elements: 54,55, 56, 57, 43, 44, 45, 46, 47, 48, 49, 44, 50, 51, 52, 53 and 54, whichcloses the loop.

It can be seen that in the process of the invention, said two recycleloops have no common portions and no mixing points. Loop REC2 of thefacility of FIG. 1 is exclusively supplied with external makeup hydrogen(via line 33) which avoids polluting it with the impurities that areoften present in the loop REC1.

If excess makeup hydrogen (hydrogen-rich gas) is supplied to the loopREC2 via line 33, depending on the hydrogen requirement in reaction sepa3), the excess gas can advantageously be used as the stripping gas forcolumn 10 (evacuated from the loop REC2 via line 58), this gas beingsubstantially free of impurities. The supply of excess makeup gas to theloop REC2 results in increased purity of the gas in loop REC2 as thepurge extracts light hydrocarbons containing 1 to 4 carbon atoms andtraces of residual H₂S from said loop.

Optionally, for example if the loop REC2 functions with a quantity ofpurge gas exceeding the requirements of the stripping gas, a fraction oreven all of the purge gas for the loop REC2 can optionally be sent bymeans that are not shown to a point in the loop REC1 (for example ofline 23) without passing via the stripping column.

In the facility of FIG. 1, the two loops REC1 and REC2 are supplied withmakeup hydrogen (hydrogen-rich gas) via lines 34 and 33 respectively.

The two streams of makeup hydrogen can be of different purities. Thehydrogen supplied via line 34 to loop REC1 can optionally derive from acatalytic reformer and be of medium purity, for example between 88 and96. The hydrogen supplied via line 33 to loop REC1 can optionally andpreferably derive from a steam reforming unit followed by PSA typeseparation and be of very high purity, for example 99.9.

The two loops REC1 and REC2 could, however, be supplied from a commonline, not shown, using makeup hydrogen of identical purity.

The facility can also comprise other elements that are not shown in FIG.1, for example:

-   -   one or more quench gas lines originating from points on line 23        and supplying the reactor 7 at an intermediate position (in one        or more zones each located between two consecutive catalytic        beds);    -   one or more stripping gas supplies for the column 10 deriving        from one or more points on line 23 or line 21.

These additional lines could then be included in the loop REC1.

The facility can also comprise a line for evacuating purge gas from apoint in the recycle loop REC1 and/or a line for introducing makeuphydrogen at a point in this loop REC1 without passing via the strippingcolumn (for example at line 23).

In the same manner, the loop REC2 could comprise elements that are notshown in FIG. 1, for example one or more quench lines deriving frompoints in line 56 and supplying the reactor 48 at an intermediateposition (zones between catalytic beds). The facility can also compriseevacuating purge gas from a point in the loop REC2 without it supplyingthe loop REC1.

The scope of the invention encompasses adding and/or removing heatexchangers or equivalent equipment and/or organizing the thermalintegration of the facility in a different manner. As a non limitingexample, the exchanger 46 in loop REC2 could be the furnace 5 itself (ora portion of that furnace, in particular a portion of the furnaceconvection zone). It is also possible, and often done, to preheat thefeed for step a1) and/or the makeup hydrogen with the effluent from stepa3) and/or to preheat the recycle gas from the loop REC2 with theeffluent from step a3) and/or to recover heat from the head effluentfrom column 10 upstream of the air cooled exchanger 12.

The effluent from the reactor for step a1) can be cooled in an exchanger(or a plurality of exchangers). Cooling (for all of the exchangers if aplurality of exchangers is used) can be substantial, for example usuallyin the range about 90° C. to 200° C. in the first variation of theprocess of the invention. It can also be limited, usually to at most 90°C., generally at most 70° C. in the second variation of the process ofthe invention, to keep the temperature high at the inlet to thestripping column. This effluent can also be reheated in a limited mannerin a furnace prior to supplying the stripping column. The strippingliquid effluent (or heavy stripping liquid effluent) can optionally bereheated in a heat exchanger and/or a furnace before supplying thereactor for reaction step a3), or it can be cooled in a limited mannerprior to supplying the reactor. The stripping column 10 can alsocomprise a reboiler for the liquid at the column bottom, not shown inFIG. 1. In some cases, the effluent from the reactor 7 for step a1) canbe supplied directly to the stripping column (with no heat exchange)and/or the reactor for step a3) can be directly supplied (with no heatexchange) by the stripping liquid effluent.

The skilled person could also use other heat exchanges between aplurality of streams moving in the facility, depending on the respectivetemperatures of the different streams .

The scope of the invention also encompasses modifying the position ofthe H₂S purification equipment 17, that equipment then being locateddownstream of the compressor 22 (on line 23).

The scope of the invention also encompasses one or each of the tworeaction steps a1) and a3) being carried out not in one but in two oreven more reactors in series, optionally with intermediate adjustment ofthe temperature, or if one reactor comprised a plurality of reactionzones in series, with identical or different catalysts.

The hydrotreatment reactors (7, 48) are typically reactors with a fixedcatalytic bed and a downflow for the gas and liquid. The scope of theinvention encompasses whether one or more of the reactors is of afurther type or a plurality of other types, in particular of the movingbed type or an ebullated bed type (because of introduction of the gas)or a fluidized bed type (fluidized by the recycle gas), or with a fixedor moving bed in upflow mode for the gas and downflow mode for theliquid.

FIG. 2 shows a flowchart for a further hydrotreatment facility forcarrying out a process according to a second variation of the process ofthe invention, using the same reference numerals for elements common toFIGS. 1 and 2:

The first difference with the facility of FIG. 1 concerns the pumpingmeans linked to the pressures of the different reaction steps. Incontrast to the facility of FIG. 1, the facility of FIG. 2 uses apressure in step a3) that is lower than in step a1). There is thus noneed for a pump to transfer liquid stripping effluent via line 41. Incontrast, a pump 60 is used to transfer at least a portion of thehydrotreated liquid fraction moving in line 61 to the step a5) forcontact with the recycle gas of loop REC1, via line 59. A furtherportion of the hydrotreated liquid fraction can optionally be evacuateddirectly downstream (without contact) via line 62.

A further difference concerns the point for removing purge gas(optional) evacuated via line 58. This point is displaced downstream ofthe recycle compressor 55 to facilitate return of the purge gas to theloop REC1 (the pressure balance being different in the facility of FIG.2). If the pressure in the loop REC2 in particular at the discharge ofthe compressor 55 is lower than that at all points in the loop REC1, itis generally preferable not to sent the purge gas from the loop REC2 tothe loop REC1, which would necessitate a supplemental compressor (exceptif a common compressor were to be used, which would be necessary for afurther use, for example to supply a makeup gas to the loop REC1).

Finally, the loop REC2 comprises, disposed on line 56, optionalequipment 61 for purifying the recycle gas to eliminate traces of H₂S ifnecessary, for example a zinc oxide adsorbent bed. This bed canoptionally operate at a higher temperature than the outlet temperaturefor the recycle compressor by using a reheater, not shown. The adsorbentbed 61 could also be integrated into the reactor 48.

In the case in which the catalyst for step a3) is not or is veryslightly sensitive to water, for example with the conventional catalystwith no noble metal, the optional equipment 61 for purifying the recyclegas can comprise a washing column using an aqueous amine solution.

These variations in the process with H₂S purifier 61 are not connectedwith staggering the pressures between steps a1) and a3); thus, they canalso be used in the facility of FIG. 1.

The other elements of FIG. 2 are identical to those of FIG. 1. For thefacility of FIG. 2, it is also possible to use options or technicalmodifications such as those described for the facility of FIG. 1 withoutdeparting from the scope of the invention.

In general, the facility for hydrotreatment of a hydrocarbon feed tocarry out the process of the invention comprises:

-   -   a first hydrogen recycle loop REC1, said loop comprising at        least one first hydrotreatment reactor 7 connected downstream to        a column 10 for pressurized stripping of the liquid effluent        from the reactor using a hydrogen-rich gas, the head of the        column 10 being connected to a means 12 for cooling and partial        condensation of the gas stream deriving from the column 10, said        cooling and partial condensation means being connected        downstream to a first gas/liquid separator 14, itself connected        to the intake of a first recycle compressor 22, the discharge        from said first compressor being connected to a first        hydrotreatment reactor 7;    -   a second hydrogen recycle loop REC2, separate from the loop        REC1, said loop comprising at least one second hydrotreatment        reactor 48, said reactor being connected upstream to the bottom        of the stripping column 10, for hydrotreatment of the liquid        effluent issuing from the bottom of the stripping column 10, and        connected downstream to a second gas/liquid separator 53, itself        connected to the intake for a second recycle compressor 55, the        discharge from said second compressor being connected to the        second hydrotreatment reactor 48.

Preferably, the loop REC2 is supplied with hydrogen via one or moresupply means 33, each of said supply means 33 being connected upstreamexclusively to one or more external sources of hydrogen.

The facility can also comprise at least one line 58 for supplying astream of purge hydrogen from the loop REC2 to the loop REC1, said linebeing connected upstream to a point in the loop REC2 and downstream to apoint in the loop REC1.

The facility can also comprise a first means 34 for supplying anexternal makeup hydrogen stream with a relatively low purity to loopREC1, and a second means for supplying an external makeup hydrogenstream with a relatively high purity to loop REC2.

Preferably, the stripping column comprises above its supply point arectification zone having a separation efficiency of at least 1theoretical plate, for example in the range 2 to about 20 theoreticalplates, and usually in the range 4 to 15 theoretical plates, limitsincluded.

The facility can also comprise a line 27 for downstream evacuation of alight liquid stripping fraction, said line being connected upstream tothe first gas/liquid separation means 14.

The various supply and/or evacuation means mentioned in this descriptiontypically comprise at least one line and can also comprise one or morevalves and/or measuring means and/or regulating means, for example forthe flow rate and/or temperature.

EXAMPLES

The following examples provide non limiting explanations of theoperating conditions used in the process of the invention:

Example 1

Feed treated: light cycle oil (LCO) with the following characteristics:

-   -   distillation interval (5%-95% distilled): 205-347° C.;    -   density: 0.91;    -   sulphur content: 4000 ppm;    -   nitrogen content: 800 ppm;    -   aromatics content: 60% by weight;    -   cetane index: 28;    -   purity of makeup hydrogen: 92.5.

Operating Conditions in First Step a1) and First Loop REC1:

-   -   catalyst: HR448, Co—Mo on alumina, sold by AXENS (formerly        PROCATALYSE);    -   mean reactor temperature: 380° C.;    -   reactor outlet pressure: 9.8 MPa;    -   H₂ partial pressure, reactor outlet: 6.2 MPa;    -   HSV (hourly space velocity): 0.6 h⁻¹;    -   hydrogen (reactor inlet+quench): 625 Nm³/m³ of feed;    -   hydrogen consumed in step a1): 2.03% by weight with respect to        feed;

Operating Conditions in Step a2):

-   -   stripping column inlet temperature: 220° C.;    -   stripping column inlet pressure: 9.5 MPa;    -   number of theoretical plates above supply: 9;    -   number of theoretical plates below supply: 15    -   reflux ratio with respect to hydrocarbon feed for column: 0.25        kg/kg/;    -   stripping hydrogen: flow rate corresponding to 95% of hydrogen        consumed in first step;

Operating Conditions for Second Reaction Step a3) and Second Loop REC2:

-   -   catalyst: LD 402 catalyst, platinum on alumina, sold by AXENS        (formerly PROCATALYSE). This catalyst has a high hydrogenation        efficiency and step a3) is essentially hydrogenation;    -   mean reactor temperature: 300° C.;    -   reactor outlet pressure: 8.5 MPa;    -   partial pressure of H₂, reactor outlet: 6.8 MPa;    -   HSV: 7.0 h⁻¹;    -   hydrogen (inlet+quench): 700 Nm³/m³ of feed for step a3);    -   hydrogen consumed in step a3): 1.10% with respect to feed for        step a1).

Results:

At the end of the first step, the feed contained 10 ppm of sulphur, 5ppm of nitrogen and 27% by weight of aromatics.

At the end of the stripping step, about 99% by weight of the fraction offeed boiling above 150° C. was recovered from the bottom of thestripping column. The facility of Example 1 thus functioned inaccordance with the first variation of the process of the invention.

The final product, fractionated at the facility outlet, had thefollowing characteristics:

Density: 0.84;

sulphur content: 5 ppm;

nitrogen content: 1 ppm;

aromatics content: 5% by weight;

cetane index: 48;

distillation interval(5-95%): 195-337° C.

Example 1 used a higher pressure in the first reactor than in the secondreactor. This example can thus be carried out in a facility of the typeshown in FIG. 2. The preceding results correspond to a facilityflowchart such as that shown in FIG. 2 but retaining step a5) forcontact upstream of the gas/liquid separator 20 as indicated in FIG. 1.In example 1, all of the relatively light hydrocarbon liquid phaseseparated in the gas/liquid separator 14 was used as a reflux in thestripping column.

The purity of the hydrogen at the outlet from the reactor for the firststep a1) was 63.26, while it was 80 at the outlet from the reactor forthe second reaction step a3). This resulted in a partial pressure of 6.8MPa at the outlet from the reactor for step a3) that was higher than thepartial pressure of hydrogen at the outlet from the reactor for thefirst step a1), which was 6.2 MPa, while the order was reversed for thetotal pressures.

The feed supplied to step a3) was completely free of impurities such asH₂S, NH₃, H₂O (less than 5 ppm for each of these compounds) and morethan 99% by weight of the light hydrocarbons containing 1 to 4 carbonatoms present at the outlet from step a1).

Example 2

Feed: identical to that of Example 1.

The facility was designed to obtain the same quality of feed after thefirst step and the same final product as in the facility of Example 1.This facility also carried out stripping with column bottom recovery ofabout 99% by weight of the fraction boiling above 150° C., and alsofunctioned in accordance with the first variation of the process of theinvention.

Purity of makeup hydrogen: 99.999.

Operating Conditions in First Step a1) and First Loop REC1:

-   -   catalyst: HR448, Ni—Mo on alumina, sold by AXENS (formerly        PROCATALYSE);    -   mean reactor temperature: 380° C.;    -   reactor outlet pressure: 7.3 MPa;    -   H₂ partial pressure, reactor outlet: 5.8 MPa;    -   HSV (hourly space velocity): 0.55 h⁻¹;    -   hydrogen (inlet+quench): 625 Nm³/m³ of feed;    -   hydrogen consumed in step a1): 2.03% by weight with respect to        feed;

Operating Conditions in Step a2):

-   -   stripping column inlet temperature: 220° C.;    -   stripping column inlet pressure: 7.0 MPa;    -   number of theoretical plates above supply: 9;    -   number of theoretical plates below supply: 15    -   reflux ratio with respect feed to facility (liquid reflux/step        a1 feed): 0.25 kg/kg/;    -   stripping hydrogen: 95% of hydrogen consumed in step a1);

Operating Conditions for Second Reaction Step a3) and Second Loop REC2:

-   -   catalyst: LD 402 catalyst, platinum on alumina, sold by AXENS        (formerly PROCATALYSE);    -   mean reactor temperature: 300° C.;    -   reactor outlet pressure: 7.6 MPa;    -   partial pressure of H₂, reactor outlet: 7.2 MPa;    -   HSV: 7.5 h⁻¹;    -   hydrogen (inlet+quench): 700 Nm³/m³ of feed for step a3);    -   hydrogen consumed in step a3): 1.10% with respect to feed for        step a1).

Example 2 used a higher pressure in the second reactor than in the firstreactor. Thus, this example could be carried out in a facility of thetype shown in FIG. 1.

The hydrogen purity at the outlet from the reactor for the first stepa1) was 79.45, while it was 94.73 at the outlet from the reactor for thesecond reaction step a3).

This example is not limiting and it is possible, for example, to havelower pressures in step a1), for example absolute pressures in the range3.8 to 6.2 MPa, while the pressure in step a3) could be higher by 1.2 to4.5 MPa, for example at that of step a1). If the pressure in step a1) isrelatively low, and the feed for step a3) still contains substantialtraces of sulphur, it is possible to use in step a3) catalysts with twonoble metals, or compounds of noble metals, for example of theplatinum/palladium on alumina type and/or any type of catalyst that isresistant to traces of sulphur (and/or eliminate said traces of sulphurin the loop REC2).

For a variety of feeds and product specifications, the process of theinvention can very effectively eliminate all of the pollutants presentin the first hydrotreatment step and allows the us of the best availablecatalysts in an optimal manner in the second step (high partial pressureof hydrogen and minimal impurities content), with high energeticefficiency, without necessitating consumption of stripping vapour.

1. A process for hydrotreating a hydrocarbon feed containingsulphur-containing compounds, comprising the following steps: a firststep a1) for hydrotreatment in which said feed and excess hydrogen arepassed over a first hydrotreatment catalyst to convert at least aportion of the sulphur contained in the feed into H₂S; downstream ofstep a1), a step a2) for stripping the at least partially desulphurizedfeed from step a1) in a pressurized stripping column, using at least onehydrogen-rich stripping gas, to produce at least one hydrogen-richgaseous stripping effluent and at least one liquid stripping effluent,the gaseous stripping effluent being at least partially compressed andrecycled to the inlet to the first step a1) using a first recycle loopREC1; downstream of step a2), a second hydrotreatment step a3) in whichthe stripped liquid effluent and excess hydrogen are passed over asecond hydrotreatment catalyst, the effluent from said step a3) beingfractionated, in a gas/liquid separation step a4), into a hydrogen-richgaseous fraction and a hydrotreated liquid fraction, said hydrogen-richgaseous fraction being at least partially compressed and recycled to theinlet to step a3) using a second recycle loop REC2 separate from theloop REC1.
 2. A process according to claim 1, further comprising a stepa5) for bringing at least a portion of said hydrotreated liquid fractioninto contact with at least a portion of the gaseous stream moving inloop REC1, the effluent from said step a5) being separated into a liquidcontacting effluent and a gaseous contacting effluent, at least aportion of said gaseous contacting effluent being recycled to the loopREC1.
 3. A process according to claim 1, in which the loop REC2 issupplied with hydrogen independently of the loop REC1, via one or morestreams of hydrogen exclusively constituted by streams of makeuphydrogen external to the loop REC1.
 4. A process according to claim 1,further comprising one or more treatments carrying out at least partialelimination of the H₂S contained in the recycle gas moving in the loopsREC1 and REC2, in which each of the treatment or treatments isconstituted by a combination of a step for bringing at least a portionof the recycle gas moving in the loop REC1 into contact with a liquidhydrocarbon fraction to carry out limited absorption of H₂S by saidliquid hydrocarbon fraction followed by a step for gas/liquid separationof the mixture issuing from said contact and direct evacuation of atleast a portion of the separated liquid.
 5. A process according to claim1, in which the stripping step a2) is carried out in a stripping columncomprising a section for rectification of stripping vapours using aliquid reflux.
 6. A process according to claim 1, in which the vapoursfrom the head of the stripping column are cooled to condense arelatively light, substantially desulphurized liquid hydrocarbon phase,the cooled vapours are separated from said relatively light hydrocarbonliquid phase in a reflux separator drum, a fraction of said relativelylight hydrocarbon liquid phase is taken off and used as the liquidreflux for the stripping column, and at least a portion of the residualfraction of the relatively light hydrocarbon liquid phase is evacuateddirectly downstream.
 7. A process according to claim 1, in which a flowof excess makeup hydrogen compared with the hydrogen requirements ofstep a3) is supplied to step a3), a stream of hydrogen-rich purge gas isextracted from the loop REC2 and said stream is sent to at least onepoint in the loop REC1.
 8. A process according to claim 7, in which atleast a portion of said purge gas is used as the stripping gas for stepa2).
 9. A process according to claim 1, in which the hydrotreatment stepa3) is carried out with at least one catalyst comprising at least onenoble metal or a noble metal compound selected from the groupconstituted by palladium and platinum.
 10. A process according to claim9, in which the degree of desulphurization in step a1) is selected sothat the feed for step a3) has a sulphur content that is compatible withthe sulphur tolerance threshold of the catalyst for step a3).
 11. Aprocess according to claim 1, in which the purity Pur2 of the hydrogenin the recycle loop REC2 is greater than the purity Pur1 of the hydrogenin the recycle loop REC1.
 12. A process according to claim 1, in whichthe loops REC1 and REC2 are supplied with separate streams of makeuphydrogen with different purities, the purity of the makeup hydrogenstream supplying the loop REC2 being greater than that of the makeuphydrogen stream supplying the loop REC1.
 13. A process according toclaim 1, in which the pressure in step a3) is greater by at least 1.2MPa and at most 12 MPa than that in step a1).
 14. A process according toclaim 1, in which the pressure in step a3) is lower by at least 0.1 MPathan that in step a1), and the partial pressure of hydrogen in step a3)is higher than the partial pressure of hydrogen in step a1).
 15. Ahydrocarbon cut from the group formed by gasoline, jet fuel, kerosene,diesel fuel, gas oil, vacuum distillate and deasphalted oil, containingat least one fraction hydrotreated using the process according toclaim
 1. 16. A facility for hydrotreating a hydrocarbon feed to carryout the process according to one of claims 1 to 15, comprising: a firsthydrogen recycle loop REC1, said loop comprising at least one firsthydrotreatment reactor 7 connected downstream to a column 10 forpressurized stripping of the liquid effluent from the reactor using ahydrogen-rich gas, the head of the column 10 being connected to a means12 for cooling and partial condensation of the gas stream deriving fromthe column 10, said cooling and partial condensation means 12 beingconnected downstream to a first gas/liquid separator 14, itselfconnected to the intake of a first recycle compressor 22, the dischargeof said first compressor being connected to a first hydrotreatmentreactor 7; a second hydrogen recycle loop REC2, separate from the loopREC1, said loop comprising at least one second hydrotreatment reactor48, said reactor being connected upstream to the bottom of the strippingcolumn 10, for hydrotreatment of the liquid effluent issuing from thebottom of the stripping column 10, and connected downstream to a secondgas/liquid separator 53, itself connected to the intake for a secondrecycle compressor 55, the discharge from said second compressor beingconnected to the second hydrotreatment reactor
 48. 17. A facilityaccording to claim 16, in which the loop REC2 is supplied with hydrogenvia one or more supply means 33, each of said supply means 33 beingconnected upstream exclusively to one or more external sources ofhydrogen.
 18. A facility according to claim 16, comprising at least oneline 58 for supplying to loop REC1 a stream of purge hydrogen from theloop REC2, said line being connected upstream to a point in the loopREC2 and downstream to a point in the loop REC1.
 19. A facilityaccording to claim 16, comprising a first means 34 for supplying anexternal stream of makeup hydrogen of relatively low purity to the loopREC1, and a second means 33 for supplying an external stream of makeuphydrogen of relatively high purity to the loop REC2.
 20. A facilityaccording to claim 16, in which the stripping column comprises, aboveits supply point, a rectification zone having a separation efficiency ofat least 1 theoretical plate.
 21. A facility according to claim 16,comprising a line 27 for downstream evacuation of a light strippingliquid fraction, said line being connected upstream to the firstgas/liquid separator 14.